An intake air amount estimation device for estimating an amount of an air sucked into a combustion chamber of an internal combustion engine, is disclosed in the Patent Document 1. In this intake air amount estimation device, the intake air amount is estimated by using a model formula which is obtained using physical laws.
In the intake air amount estimation device disclosed in the Patent Document 1, the pressure in the combustion chamber (hereinafter, this pressure will be referred to as —cylinder pressure—) is used in order to estimate the intake air amount. Then, the cylinder pressure includes the cylinder pressure at the time when an intake valve closes. Therefore, in order to exactly estimate the intake air amount in the intake air amount estimation device disclosed in the Patent Document 1, the cylinder pressure at the time when the intake valve closes, should be exactly obtained.